A photothermographic material has long been proposed. Photothermographic materials are described in, e.g., U.S. Pat. Nos. 3,152,904 and 3,457,075, and B. Shely, "Thermally Processed Silver System", Imaging Processes and Materials, Neblette, 8th ed., Sturge, V. Walworth, A. Shepp, page 2, 1996.
In general, a photothermographic material comprises a photosensitive layer having a catalytically active amount of a photocatalyst (e.g., silver halide), a reducing agent, a reducible silver salt (e.g., organic silver) and a toning agent for controlling the tone of silver dispersed in a binder matrix.
A photothermographic material is imagewise exposed to light, and then heated to a temperature as high as, e.g., not lower than 80.degree. C. so that the silver halide or reducible silver salt (which acts as an oxidizing agent) and the reducing agent undergo redox reaction with each other to form a black silver image. The redox reaction is accelerated by the catalytic action of a silver halide latent image which has been generated upon exposure. Therefore, the black silver image is formed on the exposed area.
The heat development process doesn't require the use of a processing solution as used in wet development process and thus is advantageous in that it can be effected simply and rapidly. However, the wet development process is still mostly used to form an image in the art of photography. This is because the heat development process has some problems unresolved unlike the wet development process.
It is known that a photographic light-sensitive material has constituent layers colored for the purpose of inhibiting irradiation. Irradiation is a phenomenon that light which has entered into a photosensitive layer undergoes reflection or diffusion inside the emulsion to make even the periphery of the normal image to be exposed to light. The use of such an anti-irradiation technique makes it possible to enhance sharpness. However, a photothermographic material is disadvantageous in that it is liable to increased development fog or discoloration during storage or, even if no discoloration occurs during storage, exhibits a reduced transparency or absorbance if it comprises a colorant incorporated therein.
If such an anti-irradiation dye is incorporated in the photosensitive layer, it can have an effect on the heat development, causing an increase in the generation of fog.
On the other hand, as a technique for inhibiting fog there has heretofore been practiced the use of mercury compound. However, such a mercury compound is not desirable from the environmental standpoint of view. Thus, alternative techniques have been studied. As such compounds there have been proposed various compounds such as organic halogen compound.
However, such a fog inhibitor other than mercury compounds is not sufficient for the effect of inhibiting fog exerted by the incorporation of the foregoing irradiation inhibitor in the photosensitive layer.